(a) Field of the Invention
This invention relates to an apparatus and method for agitating the pulp suspension on the forming fabric of a paper making machine and particularly for disturbing the initial fiber mat in the region of the forming board and/or for deflocculating the pulp suspension in the region of dewatering foils.
(b) Description of the Prior Art
In the paper forming section of a paper making machine pulp stock, which is a thin suspension of fibers and fillers containing generally about 99.5% water, is flowed or ejected from a headbox slice at the upstream end of the section onto the surface of a moving endless screen belt, called a forming fabric, which is made of woven metal or plastic filaments. The forming fabric passes over various fabric support devices which withdraw some of the water from the pulp stock leaving on the fabric a thin, self-supporting formation of matted fibers. This mat is lifted off the fabric at the downstream end of the section and is passed through a press section where more water is removed by mechanical means and then through a dryer section where dewatering is completed by evaporation.
In the conventional Fourdrinier type machine the pulp stock is discharged from the slice onto a nearly horizontally disposed forming fabric and is dewatered as the fabric runs over a forming board, table rolls and/or foils and suction boxes. In recent years twin wire machines have been gaining popularity because of greater speed capability and requiring less space and energy. There are several known types, each involving the ejection of pulp stock onto a forming fabric as it converges with another fabric running in the same direction and at the same speed. The fabrics are arranged to converge until they run together in a dewatering zone with the layer of pulp sandwiched between them. In some of these machines the dewatering zone includes a forming board, foils and an arcuate or curved shoe which may be adapted to embody the present invention.
Regardless of which type of paper machine is used the quality of the finished paper depends to a large extent on the uniformity of the suspension of fibers in the pulp stock being maintained while water is being withdrawn and until the fibers are no longer in suspension but have formed a fibrous mat. A well distributed and uniform pulp discharge from the slice is readily obtainable by thorough mixing in the headbox and by micro-scale turbulence produced within the discharge nozzle of the slice. However, this turbulence decays rapidly and dissipates before it can be effectively used at or just beyond the point of impingement of the pulp discharge on the forming fabric. Thus, unless specific means are employed to produce continual disturbance in the stock suspension, the fibres of the pulp will immediately commence to agglomerate and form flocs which reduce retention and drainage throughout the formation zone and this results in poor fiber formation in the finished sheet of paper.
Continual disturbance of fibers in the stock in the initial stages is also beneficial in overcoming what is known as "pressure formation" where too much initial drainage occurs as a result of the combined effect of the vacuum created by breast roll discharge action and the impact of the pulp jet impinging on the fabric at relatively steep angles. This causes premature embedment of a mat of fibers into the forming fabric and significantly decreases subsequent drainage and deflocculation.
Many methods have been proposed to agitate the fibers of the stock after it has been deposited on the fabric. A well known method, which is applicable to fairly small, slow running paper machines, is to mechanically shake the whole upstream frame of the machine. In cases where it is not practical to shake a massive framework, smaller components of the machine such as table rolls, or foils have been caused to shake or vibrate. Other methods involve the development of a succession of pressure pulses in the pulp suspension and these include rotating a roll having a textured surface close to the underside of the forming fabric as in U.S. Pat. No. 4,306,934 (Seppanen), interrupted foiling action as in U.S. Pat. No. 3,573,159 (Sepall) and U.S. Pat. No. 3,874,998 (Johnson), and intermittent foiling action at the forming board as in U.S. Pat. No. 3,598,694 (Wiebe). Still other methods involve the creation of intermittent lateral flow in the stock suspension by induced uneven drainage as in U.S. Pat. No. 1,917,098 (Cofrin) or by upwardly directed air jets at the forming board as in the U.S. Pat. No. 3,149,026 (Hornbostel). Still other methods include rapid vertical oscillation of the forming fabric as it travels through the forming zone over corrugated suction box covers as in U.S. Pat. No. 3,102,066 (Justus) or over specially designed or positioned foil blades that promote vertical undulations as in U.S. Pat. No. 3,922,190 (Cowan) and U.S. Pat. No. 4,140,573 (Johnson).
A disadvantage inherent in the above mentioned prior art is that agitation is not produced close enough to the point where formation commences as the pulp stock impinges on the forming fabric so that initial sealing of the fabric by fibers of stock is prevented. Another disadvantage of the prior art is that agitation is produced only on a macro-scale and the turbulence it produces is not fine scale enough to augment and maintain the micro-scale turbulence in the pulp stream issuing from the slice. Another disadvantage of the prior art is that the various devices are of fixed configuration and are not easily changed to meet operational changes required by variations of pulp stock. Still another disadvantage of the prior art is that components of the machine which initiate agitation are subject to wear and, over a period of time, the turbulence created by them is not constant.